Spatial and temporal scaling of intercellular CO ₂ concentration in a temperate rain forest dominated by Dacrydium cupressinum in New Zealand

Seven methods, including measurements of photosynthesis (A) and stomatal conductance (g _s), carbon isotope discrimination, ecosystem CO ₂ and water vapour exchange using eddy covariance and the use of a multilayer canopy model and ecosystem Keeling plots, were employed to derive estimates of intercellular CO ₂ concentration (C _i) across a range of spatial and temporal scales in a low productivity rain forest ecosystem dominated by the conifer Dacrydium cupressinum Lamb. in New Zealand. Estimates of shoot and canopy C _i across temporal scales ranging from minutes to years were remarkably similar (range of 274-294 μmol mol ^-1). The gradual increase in shoot C _i with depth in the canopy was more likely attributable to decreases in A resulting from lower irradiance (Q) than to increases in g _s due to changes in air saturation deficit (D). The lack of marked vertical gradients in A and g _s at saturating Q through the canopy and the low seasonal variability in environmental conditions contributed to the efficacy of scaling C _i. However, the canopy C _i estimate calculated from the carbon isotope composition of respired ecosystem CO ₂ (δ ¹³C _R; 236 μmol mol ^-1) was much lower than other estimates of canopy C _i. Partitioning δ ¹³C _R into four components (soil, roots, litter and foliage) indicated root respiration as the dominant (> 50%) contributor to δ ¹³C _R. Variable time lags and differences in isotopic composition during photosynthesis and respiration make the direct estimation of canopy C _i from δ ¹³C _R problematic.